Initialization device for echo cancelling device and application thereof to remote echos

ABSTRACT

The present invention relates to echo cancelling techniques and more particularly to an initialization device for an echo cancelling system. The invention comprises means for converting a pseudo-random sequence (u k ) of length N into a sequence of emitted samples (e k ) such that each sample e k  is derived from u k  by e k  =2 u k  -1, multiplying each sample u.sub.(N-M+1+k)mod N with each received sample s N+k  and adding the results k varying between 0 and N-1. The result at instant kT, to within a multiplicative constant, of said additions is then H M  (k), one of the N coefficients at the initial conditions of said echo cancelling device, M being between 0 and N, and k varying from 0 to N-1, with H M  (k)=H M  (k-1)+s N+k . u.sub.(N+k+1-M)mod N. The invention is applicable to the pulse response of any signal.

BACKGROUND OF THE INVENTION

The present invention relates to simultaneous two-way data transmissionon transmission channels, and more particularly to the echo cancellationtechnique.

Echo cancelling devices provide an estimate of the echo and subtract itfrom the perturbated signal.

This estimate of the echo signal is usually made at the instants ofsampling by adjusting the coefficients C_(k) of the echo cancellingdevice. This adjustment is usually made by the gradient algorithm. Sucha technique has been described for example in the article by Muller inIEEE Transactions on Communications, Vol. 24, No. 9, Sept. 76, pages956-962.

Adjustments employing the gradient algorithm are simple to carry out butthe convergence of the echo cancelling device using this process is notvery rapid.

It is an object of the present invention to improve the rapidity of theconvergence of the echo cancelling devices depending on the types ofexploitation used, by employing an initialization device. Moreover, theinitialization device according to the invention enables the remote echoto be located and is independent of the algorithm employed.

The initialization device according to the invention allows aninitialization of any echo cancelling device according to a proceduresuch that each modem successively emits, and initializes its own echocancelling device in the absence of a signal coming from the remote end.According to such a procedure, each modem emits an appropriate sequenceenabling it to cancel its echo, the remote modem not emitting.

SUMMARY OF THE INVENTION

The initialization device for an echo cancelling device according to theinvention comprises means for forming N coefficients for the initialconditions in the cancelling device. These means comprise essentiallymeans for converting a pseudo-random sequence u_(k) of length N into asequence of samples e_(k) such that each sample e_(k) is derived fromu_(k) by e_(k) =2 u_(k) -1, means for emitting said e_(k) on-line, meansfor receiving samples s_(N+k) at reception, means for multiplying eachsample s_(N+k) received with N samples u.sub.(N-M+1+k)mod N issuing fromthe cell A'(M) of a shift register, said u.sub.(N-M+1+k) being derivedfrom e_(N-M+1+k) by inverse conversion, i.e. u_(k) =(e_(k) +1)/2, andmeans for adding said N results of multiplication, for all the k's, whenk varies from 0 to N-1, the result of said addition being H_(M), one ofthe N coefficients at the initial conditions, M varying from 1 to N.

The device of the invention also makes it possible to form two sequencesof samples a_(k) and b_(k) from the pseudo-random sequence u_(k) oflength N, said sequences being such that each sample a_(k) and b_(k) isderived from u_(k) by a_(k) =2 u_(k) -1 and b_(k) =2 u_(k) -1. Thesamples a_(k) and b_(k) are then modulated and emitted on-line andsamples s_(N+k) received on-line make it possible to derive therefromafter demodulation samples α_(k) and β_(k). The device of the inventioncomprises means for multiplying each sample (α_(N+k) +β_(N+k)) and(β_(N+k) -α_(N+k)) received with N samples u.sub.(N-M+1+k) mod N issuingfrom the cell A'P (M) and A'Q (M) of a shift register, saidu.sub.(N-M+1+k) being derived from e_(N-M+1+k) by inverse conversion,means for adding said results of multiplication for all the k's, when kvaries from 0 to N-1, the result of said addition being F_(p) (M) andF_(q) (M) two of the 2N coefficients at the initial conditions of saidcancelling device, M varying from 1 to N.

According to a feature, the device according to the invention is appliedto an echo cancelling device operating in baseband.

According to another feature, the length N of the pseudo-random sequenceis chosen to be greater than or equal to the length L of the pulseresponse of the echo.

According to a further feature, the initialization device makes itpossible to recognize the zero samples of the pulse response of the echoand to neglect in the echo cancelling device to update the zero samples.

The initialization device according to the invention gives to the echocancelling device a considerably rapid and reliable convergence.Moreover, the initialization sequence does not need to be standardized,it is left to the constructor to choose as a function of his needs.

The initialization device of the invention allows the echo cancellingdevice to be considerably versatile in operation, it being possible inparticular to correct the much delayed echoes, for example ontranscontinental satellite circuits or on under-water cables, and theechoes, for the person who is speaking, affected by frequency drift.

The invention will be more readily understood on reading the followingdescription with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general representation of the insertion of an initializationdevice according to the invention for echo cancellation in the case ofbaseband transmission.

FIG. 2 shows in detail the actual initialization device of FIG. 1.

FIG. 3 shows an initialization device according to the invention withmodulation for echo cancellation in baseband.

FIG. 4 shows the initialization device of FIG. 3, in detail.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to the drawings, and firstly to FIG. 1, an echo cancellingdevice 5 known per se makes an estimate of the echo from the incomingsignal in the case of baseband transmission. This incoming signal is inthe form of a sequence of complex data.

The estimate of the echo, in a cancelling device, is subtracted from thesignal received, due to subtraction means 3.

The known echo cancelling device 5 are formed for example by acoefficient updating system, using a gradient algorithm.

A hybrid circuit 2 ensures coupling of the line with emission andreception of the modem, i.e., the echo cancelling device is connected inshunt with an echo path between an emission line and a reception line.See claim 7.

As the gradient algorithm does not converge very quickly, initializationof the echo cancellation system may be fairly long.

The invention consists essentially of inserting a so-calledinitialization device 6 receiving at input the initialization sequencesignal u_(n) as well as the estimated reception signal s_(n). It is wellknown that the modems use initialization sequences, after on-lineconnection. These sequences are usually pseudo-random messages.

The properties of the pseudo-random sequences are well known. Thearticle by Williams and Sloane published in Proceedings of the IEEE,Vol. 64, No. 12, December 1976, pages 1715 to 1727, studies theirparticularities. Let u_(k) be the sequence of the samples of aninitialization sequence formed with the aid of a pseudo-random generator7. The message may be composed of (N+1)/2 "zeros" and of (N-1)/2 "ones".

According to FIG. 1, the pseudo-random generator 7 furnishes the messageu_(k) at the input of the initialization device 6. At reception, themessage is applied to the input of the initialization device 6, whichfurnishes at the output coefficients directly used by the actual echocancelling device as initial coefficients C_(k) ^(o).

Referring to FIG. 2, the message u_(k) is formed by a generator 7. Thesamples constituting the sequence u_(k) are pseudo-random of periodequal to N.

This message u_(k) is applied to the input of the actual initializationdevice 6.

The message u_(k) is converted according to the invention into a messagee_(k) formed by samples (a, -a) by means of a device 61 (firstconverting means). Each sample e_(k) is derived from the incoming sampleu_(k) by the relation e_(k) =a (2 u_(k) -1). See claim 7.

In practice, a is advantageously chosen to be equal to 1.

The message e_(k) is on the one hand emitted in-line, on the other handapplied to the input of a shift register 62 constituted by N cells A(1), A (2) . . . A (N) each introducing a delay T. Thus, cells A (1)contains e_(k), cell A (2) contains e_(k-1), cell A (M) containse.sub.(N-M+k+1) modulo N and finally register A (N) contains e_(k+1).These N samples are then applied in parallel to the input of a register63 also formed by N cells A' (1) . . . A' (M) . . . A' (N). These Ncells A' (M) form from the sample e_(N-M+k+1), samples u_(N-M+k+1) attheir output (second converting means).

Thus, cell A' (1) receives e_(k) and furnishes the sample u_(k) at theoutput, cell A' (N) receives e_(k+1) and furnishes u_(k+1) at theoutput.

Cell A' (M) generally receives sample e.sub.(N-M+k+1) mod N andfurnishes at the output sample u.sub.(N-M+k+1) mod N. These N samplesu_(k) . . . u.sub.(N+M+k-1) . . . u_(k+1) are applied to the input of Nmultiplication circuits (multipliers) (64_(i)).sub.(i=1 . . . N). Thuseach sample u_(N-M+k+1) is multiplied with the sample s_(N+k) receivedon the transmission line (reception line) at the output of thesubtraction circuit 3 (cf. FIG. 1). The N results [s_(N+k) ·u_(N-i+k+1)].sub.(i=1,N) issuing from the N circuits 64_(i) (i=1,N) arerespectively applied to the input of N registers 65_(i), which have beeninitialized to zero, via N addition circuits (66)_(i). In fact, theoperations of multiplication (due to the N multiplication circuits64_(i)) and of addition (due to the N addition circuits 66_(i)) beginonly at instant NT, i.e. from the arrival of sample s_(N), generally, tothe arrival of the samples s_(N+k), k varying from 0 to N-1.

The N addition circuits 66_(i) make it possible to accumulate at instant(N+k) T in question, the contents of the N registers 63_(i).

Thus, the respective contents of the N registers 65_(i) at instant (N+k)T are samples h_(k) (i) where:

    h.sub.k (i)=h.sub.k-1 (i)+s.sub.N+k ·[contents of A'(i)](i)

or

    h.sub.k (i)=h.sub.k-1 (i)+s.sub.N+k ·u.sub.(N+k+1-i) mod N (ii)

Then at instant (N+k+1) T, the contents of register A(i) isu.sub.(k-i+2) mod N and the sample received is s_(N+k+1), then thefollowing calculation is made and applied in register 65_(i).

    h.sub.k+1 (1)=h.sub.k (i)+s.sub.N+k-1 ·u.sub.(k-j+2) mod N (iii)

and the same applies to the N registers (65_(i)).sub.(i=1,N). A memory68 containing the constant 2/a(N+1) makes it possible to multiply eachsample h(i) of each register 65_(i), due to the N multiplicationcircuits (67_(i)).sub.(i=1,N).

The N coefficients H (i).sub.(i=1,N) issuing from the N multiplicationcircuits (67_(i)).sub.(i=1,N) are the values of the samples of the pulseresponse of the echo; they are applied to the input of the echocancelling device 5 and serve as initial coefficients.

If H_(j) are the samples of the pulse response of the echo, the signalreceived in steady state is: ##EQU1##

The values of the samples H_(j) are given by: ##EQU2##

The process of deriving U.sub.(N-M+1+k) from e.sub.(N-M+1+k) asdescribed above may generally be called inverse conversion.

Referring now to FIG. 3, the echo cancellation initialization systemaccording to the invention may also be adapted to a modulated system,the cancellation device further operating in baseband. The echocancellation device 10 receives, in the same way as in FIG. 1,coefficients F_(p) (i), F_(q) (j) furnished by an initialization device.

The initialization device 8 itself receives a pseudo-random messageu_(k) furnished by the generator 7. As will be explained with the aid ofFIG. 4, the initialization device 8 converts the message u_(k) into twomessages a_(k) and b_(k) which are each emitted on-line. These messagesa_(k) and b_(k) are on the one hand applied to the input of thecancelling device 10, on the other hand to the input of a modulator 1.The echo cancelling device 10 makes, in manner known per se, an estimateof the echo from samples a_(k) and b_(k) before modulation bymodulator 1. This estimate is modulated then substracted from the signalreceived, before a complex coherent demodulation by a demodulator 4, dueto the subtraction means 3. A differential circuit 2 ensures coupling ofthe line with emission and reception of the modem i.e., the echocancelling device is connected in shunt with an echo path between amodulator and a reception line. The samples α_(n+k) and β_(N+k) obtainedat the output of the demodulator 4 on two channels are applied to theinput of the initialization device 8. See claim 8.

FIG. 4 explains the structure and operation of the initialization device8 according to the invention. See claim 8. A coding device (firstconverting means) 81 receives the pseudo-random message u_(k) generatedby generator 7, as for the embodiment of FIG. 2. However, the device 81codes this message u_(k) on two channels in a manner known per se. Leta_(k) and b_(k) be the messages thus formed. Each sample a_(k) or b_(k)issuing from the coding system 81 is derived from the incoming sampleu_(k) by the relation a_(k) =(2 u_(k) -1) so that the a_(n) and b_(n)are formed by ±1.

In practice, a_(k) and b_(k) are chosen to be equal. Each of the twomessages (initialization device first outputs) is on the one handemitted on-line (to the emission line) after a complex coherentmodulation by means of the modulator 1 and on the other hand applied tothe input of shift registers 82 (first shift register) and 83 (secondshift register). The message a_(k) for example is applied to the inputof the register (first shift register) 82 formed by N cells AP(1) . . .AP(M) . . . AP(N) each shifting the incoming sample by a time T whilstthe message b_(k) is applied to the (second shift register) input of theregister 83 also formed by N cells AQ(1) . . . AQ(M) . . . AQ(N) eachdelaying the incoming sample by a time T. The samples a_(k), a_(k-1) . .. a_(N-M+k+1) . . . a_(k+1) coming respectively from cells AP(1) toAP(N) are respectively applied to the input of N cells AP'(1) . . .AP'(M) . . . AP'(N) of a shift register (second converting means) 84. Inthe same way, samples b_(k), b_(k-1) . . . b_(N-M+k+1) . . . b_(k+1)coming respectively from cells AQ(1) to AQ(N) are respectively appliedto the input of N cells AQ'(N) . . . AQ'(M) . . . AQ'(N) of a shiftregister (third converting means) 85. The registers (second and thirdconverting means) 84 and 85 furnish at the output, in parallel, samplesu_(k), u_(k-1) . . . u.sub.(N-M+k+1)mod N . . . u_(k+1) at the input of2N multiplication circuits (first multipliers) (86_(i)).sub.(i=1,N) and(second multipliers) (87_(j)).sub.(j=1,N).

These multiplication circuits (first and second multipliers)(86_(i)).sub.(i=1,N) and (87_(j)).sub.(j=1,N) also respectively receivethe samples (α_(N+k) +β_(N+k)), coming from addition circuit (firstadder) 92, and (β_(N+k) -α_(N+k)) coming from addition circuit (secondadder) 93. The signal α_(N+k) and the signal β_(N+k) are obtained fromthe signal s'_(N+k) obtained at the output of the subtraction circuit 3by complex coherent demodulation. To this end, the demodulator 4 iscomposed of a phase-shift circuit 41 (by π/2) and of a demodulator 42proper which furnishes the two channels in quadrature α_(N+k) andβ_(N+k). These messages α_(N+k) and β_(N+k) are therefore applied to theinput of the first adder and second adder circuits 92 and 93.

The 2N multiplication circuits, first multipliers (86_(i)).sub.(i=1,N)and second multipliers (87_(j)).sub.(j=1,N) respectively thus make itpossible to multiply the signals (α_(N+k) +β_(N+k)) and (β_(N+k)-α_(N+k)) respectively coming from first and second adder circuits 92and 93 with the contents of the i th cell and j th cell of the register84 (second converting means) and 85 (third converting means)respectively.

The result of the 2N multiplications of the (86_(i)).sub.(i=1,N) and(87_(j)).sub.(j=1,N) is respectively applied in 2N registers(90_(i)).sub.(i=1,N) and (91_(j)).sub.(j=1,N) via 2 N addition circuits(88_(i)).sub.(i=1,N) and (89_(j)).sub.(j=1,N). In the same way as forthe embodiment of FIG. 2, the 2N circuits 88_(i) and 89_(j) make itpossible to accumulate, at instant (N+k)T in question, the contents ofthe two registers 84 and 85.

The 2N registers (90_(i)).sub.(i=1,N) and (91_(j)).sub.(j=1,N) have beeninitialized to zero. The respective contents of registers(90_(i)).sub.(i=1,N), at instant (N+k)T, are samples (f_(p) (i) suchthat

    f.sub.p (i)=f.sub.p (i)+(α.sub.N+k +β.sub.n+k)·[contents of AP'(i)]

or

    f.sub.p (i)=f.sub.p (i)+(α.sub.N+k +β.sub.N+k)·u.sub.N+k+1-i)mod N

where k is between 0 and N-1.

In the same way, the respective contents of registers(91_(j)).sub.(j=1,N), at instant (N+k)T, are samples f_(q) (j) suchthat:

    f.sub.q (j)=f.sub.q (j)+(β.sub.N+k -α.sub.N+k)·u.sub.N+k+1-j)mod N

and where k is between 0 and N-1.

The contents of the 2N registers (90_(i)).sub.(i=1,N) and(91_(j)).sub.(j=1,N) are applied to the input of 2N registers(95_(i)).sub.(i=1,N) and (97_(j)).sub.(j=1,N) where the N coefficientsf_(p) (i).sub.(i=1,N) and f_(q) (j).sub.(j=1,N) are multiplied by theconstant 1/a(N+1). In fact, as 1/(N+1) is equal to a negative power of2, it is not necessary to provide a multiplication circuit. At theoutput of the N registers (95_(i)).sub.(i=1,N) or (97_(j)).sub.(j=1,N),coefficients F_(p) (i).sub.(i=1,N) and F_(q) (j).sub.(j=1,N) areobtained which are the coefficients of the echo cancelling device 10under the initial conditions. These 2N coefficients F_(p) (i) and F_(q)(j) are therefore applied to the input of the echo cancelling device 10and are the initialization device outputs.

They are also samples of the pulse response of the echo.

The embodiments of FIGS. 1 and 3 have been made in the hypothesis of anecho cancelling device in baseband.

The initialization device is more particularly interesting fortransmission systems affected by remote echoes, which is particularlyfrequent in satellite transmissions.

In fact, certain links and in particular satellite links bring about avery long delay. After initialization and more particularly in steadystate, the values of the coefficients of any echo cancelling deviceknown per se are theoretically equal to the samples of the pulseresponse of the echo returned into baseband. In this way, if this pulseresponse is constituted by two series of non-zero samples separated by aseries of zero samples of which the number depends on the delay of theremote echo, there is every interest in determining this delay in ordernot to have to calculate, adapt and store in memory, coefficients whosevalue will oscillate around zero and which would introduce noise.

Let r_(p) be the sequence of samples indicative of the pulse response ofthe echo and let us assume that, for p included between 1 and g, thesamples are non-zero; that for p between g+1 and m, the samples are zeroand that, for p between m+1 and 1, the samples are non-zero. In steadystate, any echo cancelling device known per se would make an echo signalcancellation taking into account all the samples, even those which arezero. This introduces noise on the echo cancellation.

The present invention envisages using the pulse response made by theinitialization device according to the invention for optimizing theprocessing made by the cancelling device itself.

Let us assume that the coefficients C_(k) of the echo cancelling deviceare updated according to the gradient algorithm. It is known that thestep of the algorithm is a function of the number of so-calledcoefficient samples to be processed and more precisely that this step isan inverse function of this number of samples. A number of samplestaking into account the zero samples introduce noise and reduceperformance of the cancelling device. In fact, the greater the step ofthe algorithm, the quicker is the convergence.

Thus, the initialization device according to the invention makes itpossible in steady state to determine the pulse response of the echo andvia a means to control updating, said means to order the cancellingdevice to effect updating only of the non-zero samples. To this end, thesamples of the pulse response of value greater than or equal to apredetermined threshold should be defined as non-zero.

However, in such applications, it is necessary to select theinitialization sequence in appropriate manner. In fact, the length N ofthe pseudo-random sequence u_(k) chosen for initializing the cancellingdevice must be greater than the total length L of the sequence r_(p) ofsamples indicative of the pulse response of the echo (including zerosamples).

What is claimed is:
 1. Initialization device for adaptative echocancelling device, said adaptative echo cancelling device comprising atransversal filter with N coefficients and means for constantly updatingsaid N coefficients after said initialization device has providedinitial values of said N coefficients, said echo cancelling device beingconnected in shunt with an echo path between an emission line and areception line, said initialization device having a first outputconnected to said emission line, N outputs connected to said echocancelling device, a first input connected to said reception line and asecond input connected to a pseudo random generator providing a sequenceof N binary samples u₁ . . . u_(k), . . . u_(n), said initializationdevice comprising:first converting means connected between said secondinput and said first output for deriving a sample e_(k) from everybinary sample u_(k) by the relation:

    e.sub.k =2u.sub.k -1

a shift register, connected to the output of said first convertingmeans, and comprising N cells

    A(1), . . . A(M), . . . A(N)

to store

    e.sub.k, . . . , e.sub.(N-M+k+1)modulo N, . . . , e.sub.k+1

every cell being provided with one output, a set of N second convertingmeans connected to said outputs of said cells, for deriving a sampleu.sub.(N-M+k+1) modulo N from every sample e.sub.(N-M+k+1) modulo Nstored in every said cell A(M), by the relation:

    u.sub.(N-M+k+1) modulo N= (e.sub.(N-M+k+1) modulo N +1)/2

a set of N multipliers, ever multiplier having one input connected tosaid reception line, and another input connected to an output of one ofsaid N second converting means, a set of N addition circuits, everyaddition circuit having a first input connected to an output of one ofsaid N multipliers, a set of N registers, every register having an inputconnected an output of one of said N addition circuits, and an outputconnected to another input of said one of said N addition circuits, andto one of said N outputs of said initialization device. 2.Initialization device for adaptative echo cancelling device, saidadaptive echo cancelling device comprising a transversal filter with Ncoefficients and means for constantly updating said N coefficients aftersaid initialization device has provided initial values of said Ncoefficients, said echo cancelling device being connected in shunt withan echo path between a modulator and a reception line, said modulatorprovided with an output connected to an emission line and two inputs,said reception line being connected to an input of a demodulator, saiddemodulator provided with two outputs, said initialization device havingtwo first outputs connected to said two inputs of said modulator, 2Noutputs connected to said echo cancelling device, two first inputsconnected to said demodulator outputs and a second input connected to apseudo random generator providing a sequence of N binary samples u₁, . .. , u_(k), . . . u_(N), said initialization device comprising:firstconverting means connected between said second input and said firstoutputs for deriving two samples a_(k) and b_(k) from every binarysample u_(k) by the relation:

    a.sub.k =b.sub.k =2u.sub.k -1,

said first converting means having a first output and a second output, afirst shift register, connected to the first output of said firstconverting means, and comprising N first cells

    AP(1), . . . , AP(M), . . . , AP(N)

to store

    a.sub.k, . . . , a.sub.(N-M+k+1)modulo N, . . . , a.sub.k+1

every first cell being provided with one output, a second shiftregister, connected to said second output of said first convertingmeans, and comprising N second cells

    AQ(1), . . . , AQ(M), . . . , AQ(N)

to store

    b.sub.k, . . . , b.sub.(N-M+k+1)modulo N, . . . , b.sub.k+1

every second cell being provided with one output, p1 a set of N secondconverting means, connected to said outputs of said N first cells, forderiving a sample u.sub.(N-M+k+1) modulo N from every samplea.sub.(N-M+k+1) modulo N stored in every said first cell AP(M) by therelation:

    u.sub.(N-M+k+1)modulo N =(a.sub.(N-M+k+1)modulo N +1)/2

a set of N third converting means, connected to said outputs of said Nsecond cells, for deriving a sample u.sub.(N-M+k+1) modulo N from everysample b.sub.(N-M+k+1) modulo N stored in every said second AQ(M) by therelation:

    u.sub.(N-M+k+1)modulo N =(a.sub.(N-M+k+1)modulo N +1)/2

first and second adders connected to said two demodulator outputs forproviding a sum and a difference, respectively, of said two demodulatoroutputs' quantities, a set of N first multipliers, every multiplierhaving one input connected to said first adder, and another inputconnected to an output of a different respective one of said N secondconverting means, a set of N second multipliers, every multiplier havingone input connected to said second adder, and another input connected toan output of a different respective one of said N third convertingmeans, a set of 2N addition circuits, each said addition circuit havinga first input and a second input, each first input of said 2N additioncircuits connected to an output of a different one of said N first and Nsecond multipliers, a set of 2N registers, every register having aninput connected to an output of a different one of said 2N additioncircuits, and an output connected to the second input of each respectiveaddition circuit and to the respective one of said 2N outputs of saidinitialization device.